1. Field of the Invention
The present invention relates to a mode splitter (mode separation device) and a magneto-optical disk pick-up device using the mode splitter.
2. Description of the Related Art
As a mode separation device using a flat-plate optical waveguide, a mode splitter of a 90.degree. Bragg type, a directional coupling type or the like has conventionally been known.
However, a mode splitter of the 90.degree. Bragg type has a disadvantage of being weak against a wavelength shift. Further, the mode splitter of a directional coupling type requires strict regulation and control of a coupling length and the like during the fabrication thereof, so that there are also disadvantages of poor productivity and raising a cost.
Because of the above-mentioned disadvantages, an optical integrated device, such as an optical pick-up device including a mode splitter is difficult to be put to a practical use.
In view of such problems, in recent years, a mode splitter using a flat-plate optical waveguide has been improved so as to be applicable to the optical integrated device. An example of such a mode splitter is shown in FIGS. 12 and 13. This mode splitter has two flat-plate waveguides A and B each having a uniform thickness. The waveguides A and B are combined with each other through a coupler C. The thickness of the coupler is varied to form a tapered shape in a cross section. In this mode splitter, propagation constants .beta..sub.iA, .beta..sub.iB, .beta..sub.jA, and .beta..sub.jB and an incident angle .alpha..sub.A are set in order to satisfy the relationships: .beta..sub.iA &gt;.beta..sub.iB, .beta..sub.jA &gt;.beta..sub.jB and .alpha..sub.A &lt;arcsin (.beta..sub.iB /.beta..sub.iA), .alpha..sub.A &gt;arcsin (.beta..sub.jB /.beta..sub.jA), while at least the light wave of i mode and the light wave of j mode are propagated in the same direction.
As a result, in the case where, for example, the modes i and j are respectively represented as a TE.sub.O mode and a TM.sub.O mode, the TE.sub.O mode (a TE wave) is completely reflected on the tapered coupler C and the TM.sub.O mode (a TM wave) is transmitted from the waveguide A to the waveguide B. Accordingly, the TE wave and the TM wave can be separated from each other (i.e., a deflection separation).
However, there are problems in the above-mentioned mode splitter.
That is, while performing a mode separation in the mode splitter shown in FIGS. 12 and 13, a separated light wave remains to be confined in a waveguide layer. Accordingly, it is required to provide a grating type coupler or the like in an optical integrated device including such a mode splitter for emitting a light wave from a waveguide layer in order that the light wave confined in the waveguide layer (i.e., light wave) is carried to a detection system.
Thus, the optical integrated device including the above-mentioned mode splitter has disadvantages that its structure becomes complicated and larger-sized. Moreover, while emitting the light wave from the waveguide layer by the grating coupler or the like, the coupling is damaged due to the wavelength shift or the like.
Furthermore, the light wave is diagonally incident on the tapered coupler C (see FIG. 12), and the TE wave and the TM wave are separated (deflection separation). A slight shift in an incidental angle makes a larger reflection of the TM wave from the tapered coupler C, namely, the slight shift is a great factor for changing the reflectance of the TM wave. As a result, the extinction ratio (strength of the reflection light wave/strength of the transmittance light wave) is deteriorated.